1. Field of the Invention
The present invention relates to a multi-layer band-pass filter having balanced outputs.
2. Description of the Related Art
Reductions in size and thickness of radio communications devices such as cellular phones have been strongly sought, and techniques for mounting components with higher density have been therefore required. Integration of components through the use of a multi-layer substrate has been thus proposed.
One of the components of radio communications devices is a band-pass filter for filtering reception signals. A known type of such a band-pass filter is a multi-layer band-pass filter as disclosed in the Published Unexamined Japanese Patent Application 2003-87008. This multi-layer band-pass filter comprises a resonator made up of conductor layers of a multi-layer substrate.
A conventional multi-layer band-pass filter is designed to receive and output unbalanced signals of which ground potential is the reference potential. Therefore, to give an output signal of this band-pass filter to a balanced-input amplifier, an unbalance-to-balance transformer (balun) is required for transforming an unbalanced signal to a balanced signal made up of two signals that are nearly 180 degrees out of phase with each other and have nearly equal amplitudes. It is possible to make this balun using conductor layers of a multi-layer substrate, too.
Conventionally, the above-mentioned band-pass filter and balun are designed as discrete circuits. The Published Unexamined Japanese Patent Application 2003-87008 discloses a multi-layer dielectric filter wherein a filter and a balun are integrated through the use of a multi-layer substrate.
The Published Unexamined Japanese Patent Application 2000-349505 discloses a dielectric filter which enables receiving and outputting balanced signals without using a balun. The dielectric filter comprises: a half-wave resonator having ends open-circuited or short-circuited; a quarter-wave resonator having an end short-circuited and the other end open-circuited; an unbalanced terminal coupled to the quarter-wave resonator; and two balanced terminals coupled to portions near the two open-circuited ends of the half-wave resonator, respectively.
If the band-pass filter and the balun are made as discrete circuits, the number of components is large so that there arises a problem that the circuitry including the band-pass filter and the balun suffers greater loss and has greater dimensions. Although the multi-layer dielectric filter disclosed in the Published Unexamined Japanese Patent Application 2003-87008 has the filter and the balun integrated through the use of the multi-layer substrate, the filter and the balun are discrete circuits. Therefore, this multi-layer dielectric filter is not capable of solving the above-mentioned problem.
In the dielectric filter disclosed in the Published Unexamined Japanese Patent Application 2000-349505, the two balanced terminals are located at a distance from the half-wave resonator, and coupled to the half-wave resonator through capacitance produced between the half-wave resonator and the respective balanced terminals.
One of important parameters for determining the filter characteristics is an external Q. The external Q is Q of a resistor of an external circuit connected to the resonator. The external Q affects the acuteness of the resonance property of the resonator. The magnitude of the external Q depends on the intensity of coupling between the resonator and the external circuit. Specifically, the greater the intensity of the coupling, the smaller is the external Q.
Reference is now made to FIG. 42 to describe the relationship between the capacitance of a capacitor and an external Q obtained when a signal source is connected to a resonator through the capacitor. Here, by way of example, the resonator is a quarter-wave resonator. The circuit of FIG. 42 comprises the quarter-wave resonator 501 having an end short-circuited and the other end open-circuited. An end of the signal source 503 is connected through the capacitor 502 to the open-circuited end of the quarter-wave resonator 501. The other end of the signal source 503 is grounded through a resistor 504. The resistor 504 represents resistors of the external circuit connected to the quarter-wave resonator 501 through the capacitor 502, such as an internal resistor of the signal source 503.
The external Q of the resonator 501, expressed as Qe, is given by the following equation, where the characteristic impedance of the quarter-wave resonator 501 is Z0, the capacitance of the capacitor 502 is Cc, the angular frequency of a signal outputted from the signal source 503 is ω, and the resistance of the resistor 504 is R, wherein Qc=ωCcR.Qe=(Rπ/4Z0)(1+1/Qc2)+1/Qc
As the equation shows, the greater the capacitance Cc, the smaller is the value of Qe, that is, the greater is the coupling between the resonator 501 and the signal source 503.
Once the filter characteristics such as the center frequency, the frequency band, the number of stages, and the magnitude of ripple are determined, the external Q required is determined. If the resistance R is low, it is not necessary that the capacitance Cc is high, and it is therefore relatively easy to adjust Qe. However, if the resistance R is high, high capacitance Cc is required to obtain a desired Qe. To increase the frequency band of the filter, it is required to reduce Qe. A high capacitance Cc is required, too, in this case.
According to the dielectric filter disclosed in the Published Unexamined Japanese Patent Application 2000-349505, a terminal electrode is provided on an external surface of a dielectric block, and capacitance is produced between the terminal electrode and an internal conductor. It is difficult to obtain a high capacitance in such a configuration because of the following reason. The capacitance produced between the terminal electrode and the internal conductor is proportional to the area of the terminal electrode, and inversely proportional to the space between the terminal electrode and the internal conductor. However, it is difficult to increase the area of the terminal electrode in view of the size of the dielectric filter. In addition, if the thickness of a portion of the dielectric block between the terminal electrode and the internal conductor is reduced, the ceramic of which the dielectric block is made is broken when fired. It is therefore difficult to reduce the space between the terminal electrode and the internal conductor, too.
According to the dielectric filter disclosed in the Published Unexamined Japanese Patent Application 2000-349505, it is difficult to greatly change the area of the terminal electrode and the space between the terminal electrode and the internal conductor. It is therefore difficult to adjust the capacitance produced between the terminal electrode and the internal conductor in this dielectric filter.
As described so far, it is difficult to adjust the filter characteristics, according to the dielectric filter disclosed in the Published Unexamined Japanese Patent Application 2000-349505.